Various driver circuits are known for supplying a defined current to loads such as light emitting diodes (LEDs). As LEDs are current driven electrical loads usually a current source is used for supplying the LED with current. The load current defines the brightness of the LED device which may be a single LED component or a device including a plurality of LEDs, e.g., connected in series. In order to allow for adjusting the intensity of the emitted light (i.e., the perceived brightness) a controllable current source may be used that is set to a current representing a desired brightness. In digitally controlled applications a digital-to-analog converter (DAC) may be used to set the current of the controllable current source.
Since the human eye cannot resolve high frequency brightness fluctuations of approximately 100 hertz or higher, it is known to supply the LED with a modulated (e.g., pulse width modulated or pulse density modulated, etc.) current of sufficient frequency. In this case the human eye low-pass filters the resulting (e.g., pulse width) modulated brightness of the LED, i.e., the eye can only sense a mean brightness that depends on the mean LED current which is proportional to the duty cycle of the pulse width modulation (PWM). Consequently only the mean current through a LED is relevant to the brightness perceived by the human eye. It should be noted that many other types of electrical loads may be driven in a similar way as LEDs, e.g., a generic load whose load current is to be set via modulating a constant source current. Varying the average load current by using modulation techniques are usually preferred over driver circuits which continuously vary the load current as the actual wavelength of the emitted light may vary in an undesired manner when varying the actual load current. Using modulation techniques for adjusting the average load current (and thus the perceived brightness) do not give rise to wavelength variations as the actual load current can only assume the values zero and a preset maximum load current in accordance with a modulation signal.
For the reasons outlined above, driver circuits for driving light emitting diodes or other loads often have to be designed to switch the source current rather fast which gives rise to high current gradients in the supply lines and corresponding electromagnetic emission and electromagnetic interference (EMI). However, particularly when driving LEDs a fast switching of the load is a requirement as load current values different from the desired source current may cause a change in the hue of the emitted color and thus degrade the whole operation of a connected LED device.
In order to reduce the emissions and improve the electromagnetic compatibility (EMC) while still providing a fast and precise switching of the load current there is a need for an improved driver circuit for driving LEDs or other current-driven loads.